In recent years, a technique called a phase difference method has been used as a method for obtaining a tracking control signal from an optical disc on which data are recorded in concavo-convex pits, as represented by a CD (Compact Disc) and a DVD (Digital Versatile Disc).
Patent Document 1 (Japanese Published Patent Application No. 2004-311006) discloses an example of the phase difference method.
Hereinafter, a phase error detection apparatus 3010 disclosed in Patent Document 1 will be described with reference to FIG. 30.
FIG. 30 is a block diagram illustrating the construction of the conventional phase error detection apparatus 3010.
As shown in FIG. 30, the conventional phase error detection apparatus 3010 comprises a photodetector 101 which has light-receiving elements 101a to 101d for receiving reflected light from a light spot, and outputs light currents according to the amounts of light received by the respective light-receiving elements 101a to told, first to fourth current-to-voltage converters 102a to 102d for converting the light current outputs from the photodetector 101 into voltage signals, a signal generator for generating two signal sequences whose phases mutually change according to a tracking error of the light spot, from the voltage signals obtained by the first to fourth current-to-voltage converters 102a to 102d, i.e., first and second adders 103a and 103b, analog-to-digital converters (ADC) 104a and 104b, first and second interpolation filters 105a and 105b for subjecting the inputted digital signals to interpolation, first and second zerocross point detection circuits 106a and 106b for detecting zerocross points of the first and second digital signal sequences that are interpolated by the first and second interpolation filters 105a and 105b, a phase difference detection circuit 107 for detecting a phase difference between the zerocross point of the first digital signal sequence and the zerocross point of the second digital signal sequence, and outputting the phase comparison result as a pulse corresponding to one clock, and a low-pass filter (LPF) 108 for performing band limitation on the phase comparison signal outputted from the phase difference detection circuit 107 to obtain a tracking error signal.
In this apparatus, the photodetector 101 is provided with the light-receiving elements 101a, 101b, 101c, and 101d which are separated along a tangent direction and a vertical direction of data tracks that are recorded as data pit lines on the recording medium, and each of the first and second adders 103a and 103b adds the output signals from the light-receiving elements that are positioned diagonally among the signals generated according to the amounts of lights received by the respective light-receiving elements which are outputted from the photodetector 101, thereby generating two sequences of digital signals. Further, the zerocross point is a point where a center level of an inputted digital signal intersects with a center level of a digital signal that is calculated from an average or the like of the inputted digital signal.
Next, the operation of the conventional phase error detection apparatus 3010 will be described.
Initially, the photodetector 101 receives reflected light from a light spot that is obtained by irradiating tracks on an optical recording medium (not shown) with light, and outputs light currents according to the amounts of the received light.
The light currents corresponding to the respective light-receiving elements, which are outputted from the photodetector 101, are converted into voltage signals by the first to fourth current-to-voltage converters 102a, 102b, 102c, and 102d, and the first adder 103a adds the outputs of the first and third current-to-voltage converters 102a and 102c while the second adder 103b adds the outputs of the second and fourth current-to-voltage converters 102b and 102d. 
Then, the signals outputted from the first and second adders 103a and 103b are subjected to discretization (sampling) for the respective signal sequences by the first and second ADC 104a and 104b, and converted into first and second digital signal sequences.
Thereafter, the digital signals outputted from the first and second ADC 104a and 104b are input to the interpolation filters 105a and 105b wherein interpolated data between sampling data of the digital signals are obtained, and thereafter, zerocross points at rising edges or falling edges of the interpolated two data sequences are detected by the zerocross point detection circuit 106a and 106b. As a method for interpolation, Nyquist interpolation may be adopted. Further, as a method for detecting zerocross points at rising edges or falling edges of the two data sequences, there is a method of obtaining a sign change point (+→− or −→+) in the interpolated data sequence.
In the phase difference detection circuit 107, using the information of the zerocross points outputted from the zerocross point detection circuits 106a and 106b, a distance between the zerocross points in the waveforms of the first and second signal sequences is obtained, and a phase comparison result is outputted as a pulse corresponding to one clock on the basis of the distance between the zerocross points, and finally, band limitation is performed by the LPF 108, thereby generating a tracking error signal in a band that is needed for tracking servo control.
Next, the construction and operation of the phase difference detection circuit 107 in the conventional phase error detection apparatus 3010 will be described in more detail with reference to FIGS. 31 and 32.
FIG. 31 is a block diagram illustrating the construction of the conventional phase difference detection circuit 107.
In FIG. 31, the phase difference detection circuit 107 comprises a phase difference calculation unit 111, a pulse generation unit 112, and a data switching unit 113.
The phase difference calculation unit 111 calculates a distance between the zerocross points of the two sequences of digital signals on the basis of the zerocross information obtained by the zerocross point detection circuits 106a and 106b, and sequentially outputs the distance as a phase comparison result to the data switching unit 113.
The pulse generation unit 112 generates a pulse signal corresponding to one sampling clock in a position where zerocross occurs in each data sequence used for phase comparison, and outputs, among the generated pulse signals corresponding to the respective data sequences, a pulse signal that appears later at a point where phase comparison is performed, as a phase comparison completion signal PCC.
The data switching unit 113 outputs the phase comparison result outputted from the phase difference calculation unit 111, as a pulse corresponding to one sampling clock, at a timing of the phase comparison completion signal outputted from the pulse generation unit 112.
FIG. 32 is a diagram for explaining the operation of the phase difference detection circuit 107, and illustrates, from above, (a) a first signal sequence (phase comparison input A) outputted from the first zerocross point detection circuit 106a, (b) a second signal sequence (phase comparison input B) outputted from the second zerocross point detection circuit 106b, (c) a phase comparison completion signal PCC outputted from the pulse generation unit 112, and (d) a phase comparison result outputted from the phase difference detection circuit 107.
The two sequences of signals outputted from the first and second zerocross point detection circuits 106a and 106b, which are shown as the phase comparison inputs A (a) and B (b) in FIG. 32, are input to the phase difference calculation unit 111 and the pulse generation unit 112 of the phase difference detection circuit 107. In the phase difference calculation unit 111, phase differences Δ1, Δ2, and Δ3 are successively calculated on the basis of the zerocross data detected by the zerocross point detection circuits 106a and 106b. 
On the other hand, in the pulse generation unit 112, a pulse signal corresponding to one sampling clock is generated in a position where zerocross occurs in each of the respective data sequences to be used for phase comparison. Among the generated pulse signals corresponding to the respective data sequences, a pulse signal that appears later in the point where phase comparison is performed is outputted as a phase comparison completion signal PCC (refer to the phase comparison completion signal PCC shown in FIG. 32(c)).
Thereafter, in the data switching unit 113, on the basis of the phase comparison completion signal PCC outputted from the pulse generation unit 112, the phase comparison result PCR outputted from the phase difference calculation unit 111 is outputted at a pulse corresponding to one sampling clock (refer to the phase comparison result PCR shown in FIG. 32(d)).
FIG. 33 shows tracking error signals detected by the conventional phase error detection apparatus 3010 during CAV playback, wherein FIG. 33(a) shows a tracking error signal at the inner circumference side of the disc, and FIG. 33(b) shows a tracking error signal at the outer circumference side of the disc.
As shown in FIG. 33, an output amplitude for each pulse in the phase difference detection circuit 107 is larger at the disc inner circumference side (33(a)) having a larger number of sampling points within the same phase interval than at the disc outer circumference side (33(b)) having a smaller number of sampling points within the same phase interval. However, as shown in FIG. 32, since outputting of the phase comparison result PCR shown in FIG. 32(d) is performed by only one clock of the phase comparison completion signal PCC shown in FIG. 32(c), the time required for outputting the phase comparison result PCR is shorter at the disc inner circumference side (33(a)) having a larger number of sampling points within the same phase interval than at the disc outer circumference side (33(b)) having a smaller number of sampling points within the same phase interval.
Therefore, when the phase comparison result PCR outputted from the phase difference detection circuit 107 is subjected to band limitation by the LPF 108 to generate a tracking error signal TRE, it is possible to obtain a tracking error signal having the same amplitude at the disc inner side and the disc outer side (amplitude A and amplitude B in FIG. 33), thereby resolving linear velocity dependence of the tracking error signal during CAV playback.
As described above, in the conventional phase error detection apparatus 3010, since a tracking error can be detected by digital signal processing, it can cope with an increase in speed of an optical recording/playback device as well as an increase in density of recorded values, which cannot be achieved in tracking error detection by analog signal processing, and moreover, the constitution relating to analog signal processing can be significantly reduced, thereby realizing a small-size and low-cost optical recording/playback apparatus.